The invention relates to methods of increasing tissue plasminogen activator (tPA) mediated clot lysis and compositions for achieving increased tPA mediated clot lysis.
It is well known that surgery, delivery, major trauma and infectious diseases are associated with an increased risk of thrombosis, i.e., formation of intravascular blood clots. One may endeavor to prevent such clots by administering anticoagulants, such as heparin, coumarin derivatives, snake venom components or indanediol. In the case where thrombosis has occurred, however, use may be made of thrombolytic agents whose function is to remove the resulting blood clots from the blood vessels by dissolution (lysis).
For a better understanding of the present invention, it is noted that blood clots are composed of fibrin which has been formed from fibrinogen under action of the enzyme thrombin. The process of blood clotting (thrombosis) involves a complex system of interacting enzyme factors, each of which is converted by other enzymes from an inactive to an active form. As a result of the overall activity of this system, protein fibers called fibrin become enmeshed in a mass which curtails blood flow at the point of the thrombosis. Where such thrombosis occurs at the site of a cut, the effect is the protective reduction of blood loss through bleeding. But where such thrombosis occurs in a uncontrolled manner in major arteries supplying the lungs, brain or other vital organs, the result may be paralysis, loss of neural function, or death, unless the fibrin clot can be removed expeditiously. Examples of thrombosis in major arteries of life threatening magnitudes include cerebral thrombosis, renal thrombosis, ophthalmic artery thrombosis and thrombosis of a coronary artery. The inability to remove the fibrin clot in major arteries is the most frequent cause of mortality in the developed world.
Mammalian plasma, however, does contain an enzymatic system capable of dissolving the fibrin in blood clots. One component of the enzymatic (fibrinolytic) system consists of the enzymes referred to as plasminogen activators which convert plasminogen (an inactive proenzyme form of plasmin) into the proteolytic enzyme plasmin. Plasmin then degrades the fibrin network of the large, insoluble fibrin mass (clots) to form soluble components.
A plasminogen activator is naturally present in normal blood or is released into the blood so that the circulating blood contains, in principle, all ingredients necessary to degrade and remove an intravascular blood clot once it has been formed. One plasminogen activator, called tissue plasminogen activator (tPA), is known to exist in most human tissues. Although immunologically similar, tPA originates from different tissues and may differ from each other with respect to their molecular properties. Characteristics of tPA include greatly enhanced fibrinolytic action in the presence of fibrin, as well as high affinity for fibrin. Because of their high affinity for fibrin, the action of tPA is confined to the locality of the clot thereby reducing significantly the danger of uncontrolled hemorrhage.
In reality, however, it appears that the thrombolytic potential of the body is frequently insufficient for this purpose which means that an adequate removal of intravascular thrombi may require the use of exogenously administered thrombolytic agents. Therefore, in cases where the thrombolytic potential of the body is insufficient to remove intravascular thrombi formed, it is necessary to use exogenously administered thrombolytic agents.
It has now been discovered that a 22 amino acid peptide which was shown to have protease inhibitory activity toward certain complement proteases (G. I. Glover et al., Molec. Immunol. 25:1261 (1988) and C. S. Schasteen et al. Molec. Immunol. 25:1269 (1988)) enhances tPA mediated clot lysis. The peptide corresponds to residues 382 to 403 of Antithrombin III (ATIII) and is a member of a series of peptide serine protease inhibitors that are homologous to regions of naturally occurring serum serine protease inhibitors. The methods and compositions provided by the present invention provide a means to exogenously administer thrombolytic agents which enhance tPA mediated clot lysis, thereby helping to remove the thrombi.